Radiologic Physics, Bushong Ch 5


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Matter has mass, energy equivalence, and may also have? Electric charge
Electrostatics is the study of? Stationary electric charges
An object is said to be electrified if it has? Too many or too few electrons
In what 3 ways may electrification be created? By contact, friction, or induction
The Earth is considered to be an electric ground for what reason? It is always available to accept electric charges from an electrified object
What is the smallest unit of electric charge? The electron
What is the fundamental unit of electric charge? The coulomb
What is a coulomb equal to? C = 6 x 10^18 electron charges
The force of attraction between unlike charges or the repulsion between like charges is known as? Electrostatic force
What is given by Coulomb’s law? The magnitude of electrostatic force
What is a parallel circuit? The type of circuit that contains elements that is connected at their ends rather than lying in a line along a conductor
What is the relationship among the 3 variables in Coulomb’s law? The electrostatic force is directly proportional to the product of the electrostatic charges and inversely proportional to the square of the distance between them
Electrostatic force is very strong when objects are close, but what happens when objects separate? The electrostatic force decreases due to the inverse square law
A system that possesses potential energy is a system with? Stored energy that has the ability to do work when the energy is released
Why do electric charges have potential energy? If they are positioned close to each other they have the ability to do work when they fly apart
What is the unit of electric potential? the volt
X-ray imaging usually requires what type of voltage? 220 or higher
The volt is potential energy divided by unit charge or also defined as? Joules/Coulomb
If an electric potential is applied to copper wire, then the electrons move along the wire and this is known as? Electric current or electricity
Electrodynamics is the study of? Electric charges in motion
Why is the direction of electric current important? Because at first the assumption was that positive electric charges were conducted, but electric current is always opposite that of electron flow
What is a conductor? Any substance through which electrons flow easily
What are considered to be good electric conductors? Metal, copper, and water
What is an insulator? Any material that does not allow electron flow
What is superconductivity? The property of some materials to exhibit no resistance below a critical temperature
What are 2 examples of superconducting materials? Niobium and titanium
Does Ohm’s law apply to superconductors? No it does not
When resistance is controlled and the conductor is made into a closed path what is the result? An electric circuit
What is electric current measured in? Amperes
What is an ampere equal to? An electric charge of 1C flowing through a conductor each second
What is electric potential measured in? Volts
What is electric resistance measured in? Ohms
What are the 3 ways Ohm’s law can be written or defined? V = IR or R = V/I or I = V/R (V is the electric potential in volts, I is the electric current in amperes, and R is the electric resistance in ohms)
Electric circuits can be reduced to what 2 types? A series circuit or a parallel circuit
What is a series circuit? All circuit elements are connected in a line along the same conductor
What are the rules for a series circuit? The total resistance is equal to the sum of the individual resistances. The current through each circuit element is the same and is equal to the total circuit current. The sum of the voltages across each circuit element is equal to the total circuit voltage.
What are the rules for a parallel circuit? The sum of the currents through each circuit element is equal to the total circuit current. The voltage across each circuit element is the same and equal to the total circuit voltage. The total resistance is the inverse of the sum of the reciprocals of each individual resistance.
Cheap Christmas lights are a good example of what type of circuit? A series circuit
Electricity is the flow of electrons through? A conductor
Electrons that flow in only one direction constitute? Direct current
Electrons that flow alternately in opposite directions constitute? Alternating current
Electric power is measured in? Watts
How many kW of power does an x-ray imaging system require? 20 to 150 kW of electric power
What is one watt equal to? 1A of current flowing through an electric potential of 1V Power is equal to voltage times current (W = V x A)
What is electric power defined as? P = IV
Magnetism is a fundamental property of? Some forms of matter
Any charged particle in motion creates? A magnetic field
The magnetic field of an electron in motion is? Perpendicular to the motion of that particle
Electrons behave as if they rotate on an axis clockwise or counterclockwise, this creates a property known as? Electron spin
The lines of a magnetic field are always? Closed loops
A field that is bipolar or dipolar always has? A north and south pole
A small magnet created by an electron orbit is called? A magnetic dipole
What is created when an accumulation of many atomic magnets have their dipoles aligned? A magnetic domain
What happens if all the magnetic domains in an object are aligned? The object behaves like a magnet
What is magnetic permeability? The ability of a material to attract the lines of magnetic field intensity
What are the 3 principal types of magnets? Naturally occurring magnets, artificially induced permanent magnets, and electromagnets
What is the best example of a natural magnet? The Earth
Why does the Earth have a magnetic field? Because it spins on an axis
What are artificially produced permanent magnets usually made of? Iron
How are permanent magnets typically produced? By aligning their domains in the field of an electromagnet
How can a magnet be destroyed? By heat or banging it with a hammer, this causes the domains to knocked out of alignment
What do electromagnets consist of? Wire wrapped around an iron core
All matter can be classified according to? The manner in which it interacts with an external magnetic field
What are diamagnetic materials like? They are weakly repelled by either magnetic pole and cannot be artificially magnetized
What are examples of diamagnetic materials? Water and plastic
What are ferromagnetic materials like? They are strongly attracted by a magnet and usually can be permanently magnetized by exposure to a magnetic field
Give examples of ferromagnetic materials... Iron, cobalt, and nickel
What are paramagnetic materials like? They are very slightly attracted to a magnet and are loosely influenced by an external magnetic field
Are contrast agents employed in MRI paramagnetic? yes
What is magnetic susceptibility? The degree to which a material can be magnetized
What is hysteresis? A material that is very susceptible, but also reluctant to lose their magnetism
The physical laws of magnetism are similar to those of? Electrostatics and gravity
What is induction? The process of making ferromagnetic material magnetic
What is magnetic force proportional to? The product of the magnetic pole strengths divided by the square of the distance between them
What is the SI unit of magnetic field strength? the tesla
One tesla is equal to how many gauss? 10,000
Electricity and magnetism were viewed as separate effects until? The 19th century
A Voltaic pile is similar to? The modern battery
Any device that converts some form of energy directly into electric energy is said to be a source of? Electromotive force
What is electric potential measured in? Units of joule per coulomb, or volt
Who discovered the first link between electric and magnetic forces? Hans Oersted in 1820
What does any charge in motion induce? A magnetic field
What is a coil of wire referred to as? A solenoid
A current-carrying coil of wire wrapped around an iron core that intensifies an induced magnetic field is known as? An electromagnet
Who concluded that an electric current cannot be induced in a circuit by the presence of a magnetic field? Michael Faraday
What is an ammeter? A device that measures current
To induce a current with a magnetic field, the magnetic field cannot be? Constant, it must be changing
What is electromagnetic induction? An electric current that is induced in a circuit if some part of that circuit is in a changing magnetic field
Faraday’s law states what? The magnitude of the induced current depends on 4 factors. The strength of the magnetic field. The velocity of the magnetic field as it moves past the conductor. The angle of the conductor to the magnetic field. The number of turns in the conductor.
What is a good example of electromagnetic induction? Radio reception
What does varying magnetic field intensity induce? An electric current
What is the net effect of an electric generator? To convert mechanical energy into electrical energy
What is the type of motor used with x-ray tubes? An induction motor
The external magnetic field in an induction motor is supplied by several fixed electromagnets called? Stators
What does a transformer do? Changes the intensity of alternating voltage and current
What is the only way a transformer will operate? With a changing electric current
What is a step-down transformer? A transformer that has a turns ratio of less than 1
Does the change in current across a transformer have an inverse relationship? Yes
What is a closed-core transformer? A transformer built about a square core of ferromagnetic material
What does an autotransformer consist of? An iron core with only one winding of wire about it and it varies in both voltage and current
What is a shell-type transformer? It confines even more of the magnetic field lines of the primary winding because the secondary wire is wrapped around it and there are essentially to closed cores.
Are shell-type transformers more efficient than close-core transformers? Yes, and they are the most currently used type of transformer
What are the laws of electrostatics? Like charges repel, unlike charges attract